Cell movements and changes in cell shape often are accompanied by changes in the supramolecular form of actin. An example is the elongation of microvilli which occurs on the surface of sea urchin eggs following fertilization. Burgess and Schroeder (J. Cell Biol. 74, 1032 (1977)) have recently shown that these microvilli contain actin filaments in the form of a bundle, and Spudich and Amos (J. Mol. Biol. 129, 319 (1979)) have studied the detailed substructure of these actin filament bundles by diffraction analysis of electron microscope images. Begg and Rebhun (J. Cell Biol. 83, 241 (1979)) recently reported that cortices isolated from unfertilized eggs do not contain microfilaments, but that a dense network of microfilaments appears upon raising the pH to 7.5. This transformation can be readily studied since eggs can be obtained in large quantities and since about 20% of the actin of the egg is associated with the cortical layer (Spudich and Spudich, J. Cell Biol. 82, 212 (1979)). We will use various methods to probe the state of the actin in the cortical layer of the unfertilized egg to determine whether it is G-like, F-like, or intermediate in form. Actin from the cytoplasm, actin associated with the cell cytoplasmic surface, and that from the microvillar bundles will be characterized by various methods, including amino acid analysis to detect possible minor differences which may have resulted from enzymatic modifications. Accessory components, which may be proteins that modify the actin by binding to it or enzymatically modifying it, will be purified and characterized.